JPH04257192A - Shadow mask demagnetizing circuit for cathode-ray tube display device - Google Patents

Abstract

PURPOSE:To erase the magnetism of a shadow mask by turning ON and OFF an alternating signal, which is generated by an oscillator. alternately at intervals of a half cycle and generating an attenuating alternating magnetic field as a capacitor is discharged electrostatically. CONSTITUTION:External magnetic field data from a magnetic sensor 17 are sampled at constant intervals of time, and updated and stored in a storage circuit 20 in order. A comparator 18 compares the data stored in the circuit 20 with following data and outputs an operation signal when the difference exceeds a set value. A synchronizing circuit 16 generates a period signal whose width is an integral multiple of the period of a vertical synchronizing signal which is inputted separately from this vertical synchronizing signal and the operation signal, and sends the generated signal to a charging switch 15. In the period of the period signal, the switch 15 turns OFF and transistors 11 and 12 turn ON and OFF at intervals of a half cycle of the alternating signal generated by the oscillator 7. Then currents flows from the center tap of a demagnetizing coil 13 alternately in opposite directions and the magnetism of the shadow mask is erased as the capacitor 14 is discharged.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is a shadow mask that prevents the display screen of a cathode ray tube display device from being adversely affected by the magnetization of the shadow mask caused by an external magnetic field, and in particular, prevents color shift and deterioration of color purity in color cathode ray tubes. Regarding degaussing circuits.

0002

2. Description of the Related Art Since the shadow mask of a color cathode ray tube is a magnetic material, it is magnetized by the earth's magnetism or other external magnetic fields. Magnetization bends the trajectory of electron beams that should be incident on each of the red, green, and blue phosphors, causing color shift and deterioration of color purity. In order to eliminate this inconvenience, a demagnetizing coil is formed by winding a conducting wire around the outer periphery of the cathode ray tube display surface, and by passing an alternating current of a constant frequency through this coil while attenuating it toward zero, the magnetization of the shadow mask is demagnetized. technology is being performed.

One of the conventional degaussing circuits is shown in FIG.
The configuration shown in A) was used. Its operation is
When the power to the cathode ray tube display device is turned on, the power on timer automatically operates for a certain period of time, drives the relay 5, the relay contact 5' becomes connected, and the degaussing coil 2 is connected to the alternating power supply 3.
A demagnetizing current IL flows through the positive temperature coefficient thermistor 4.

When current flows through the PTC thermistor 4, the temperature rises due to the heat generated, and the resistance value increases with time. Therefore, the degaussing current IL becomes a damped oscillation with respect to time as shown in FIG. 3B, and degaussing is completed before the temperature of the cathode ray tube heater rises to a steady temperature and a display image appears.

[0005] Furthermore, a manual switch 6 has been provided in place of the power-on timer 6, and demagnetization can be performed at any time by turning the switch on and off. Another degaussing method uses the retrace time of vertical scanning, and uses a vertical synchronization signal to indicate that if the external magnetic field exceeds a preset strength, a damped alternating current will flow through the degaussing coil at this time. This method is synchronized with and repeated continuously.

[0006]

[Problems to be Solved by the Invention] However, the method of automatically degaussing the display device when the power is turned on is difficult to solve when the power is turned on and off infrequently, that is, when the power is turned off for a long time after the power is turned on. If there is no magnet, there is a problem that even though the magnet is demagnetized when the power is turned on, if it becomes magnetized again due to the external magnetic field after being used for a long time, it cannot be demagnetized unless the power is turned off once.

On the other hand, a method in which demagnetization can be performed at any time using a manual switch seems convenient at first glance, but this method also has the disadvantage that demagnetization cannot be performed frequently because of the heat generated by the positive temperature coefficient thermistor. In addition, in the vertical scanning retrace time utilization method, degaussing can be performed continuously while the display device is operating, but it is not possible to flow a sufficiently large alternating current during the short retrace time. There is a problem in that strong magnetization cannot be fully demagnetized.

[0008] Furthermore, in this method, demagnetization is performed when the strength of the external magnetic field exceeds a certain set value;
This poses the following problem for display devices equipped with an external magnetic field canceling function.

In a display device having an external magnetic field canceling function, even if an external magnetic field exists, a canceling coil generates a magnetic field in the opposite direction to the external magnetic field, and the external magnetic field near the electron beam trajectory of the cathode ray tube or the shadow mask is generated. is now canceled out. Therefore, in such a state, the shadow mask is not magnetized even if an external magnetic field exists.

Rather, magnetization occurs due to the delay in tracking the cancellation function when there is a change in the strength of the external magnetic field. For example, when the external magnetic field becomes stronger, the difference in the magnetic field remains until the cancellation magnetic field catches up with the strength. is applied to the shadow mask and becomes magnetized. This is because even when the external magnetic field changes to a weaker one, the canceling magnetic field becomes stronger until it catches up to the weaker one, and the magnetic field corresponding to the difference is still applied to the shadow mask.

Therefore, the demagnetization operation is not performed depending on the presence or absence of the external magnetic field or its strength itself, but is performed for a certain period of time when there is a change in the strength of the external magnetic field. The demagnetized state continues. Therefore, even if an external magnetic field exists, it would be wasteful to constantly perform demagnetization as in the retrace time utilization method. That is, in the retrace time utilization method, strong magnetization cannot be fully demagnetized, while the demagnetization operation is wastefully performed even in a demagnetized state.

SUMMARY OF THE INVENTION An object of the present invention is to provide a shadow mask demagnetization circuit that can sufficiently demagnetize strong magnetization of a shadow mask and performs a demagnetization operation only when magnetization occurs. There is a particular thing.

[0013]

[Means for Solving the Problems] In order to achieve the above object, the present invention has the following means configuration. That is, the shadow mask degaussing circuit for a cathode ray tube display device of the present invention includes: a magnetic sensor that detects the intensity of an external magnetic field; a storage device that sequentially updates and stores the value of the magnetic field intensity signal from the magnetic sensor at fixed time intervals; and a storage device. a comparator that compares the stored value with the value of the magnetic field strength at the next time, calculates the difference, and generates an operating signal when the difference value exceeds a preset value;
a synchronization circuit that receives an operating signal and a vertical synchronization signal and outputs a period signal that lasts for a predetermined number of vertical scanning cycles; a display blanking circuit that outputs a blank signal; a degaussing coil for generating a degaussing magnetic field; an alternating drive circuit that receives a period signal and alternately drives the degaussing coil during the period; serves as a power source for an alternating current that flows through the degaussing coil. A cathode ray tube display characterized by comprising: a capacitor; and a charging switch that receives a period signal and connects the capacitor to a constant voltage DC power source during periods other than the period, and disconnects the capacitor from the DC power source during the period. This is a shadow mask degaussing circuit for devices.

[0014]

[Effect] The action will be described below. In the circuit of the present invention, the intensity of the external magnetic field detected by the magnetic sensor is picked up at predetermined fixed time intervals, the previous measurement value is stored in the storage means, and compared with the next measurement value. When the difference becomes larger than a preset value, an operation signal is output for the first time, and the synchronization circuit outputs a period signal for a predetermined number of vertical scanning cycle periods using that signal, and receives this period signal. The alternating drive circuit drives the degaussing coil, while the charging switch disconnects the capacitor from the DC power source and causes an alternating current to flow through the degaussing coil using the capacitor as a power source.

As the voltage of the capacitor gradually decreases as it discharges, the current flowing through the degaussing coil becomes a damped oscillation as shown in FIG. 3(B), thereby demagnetizing the capacitor. This demagnetization is performed over a period of time that is an integral multiple of the vertical scanning period determined by the period signal,
During this time, the display blanking circuit outputs a blanking signal that cuts off the electron beam of the cathode ray tube, thereby stopping the image display. When the period signal ends, the charging switch connects the capacitor to the DC power supply again to charge the capacitor.

[0016] In this way, the degaussing function is activated over several periods of vertical scanning only when the strength of the external magnetic field changes by more than the set value and the shadow mask becomes magnetized due to the delay in tracking the external magnetic field canceling function. It can send a large demagnetizing current and can demagnetize even strongly magnetized objects.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the degaussing circuit of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing the configuration of an embodiment of the degaussing circuit of the present invention. The magnetic sensor 17 detects the strength of the external magnetic field and sends a signal to the comparator 18 and the memory circuit 20. The storage circuit 20 sequentially updates and stores magnetic field strength data at fixed time intervals.

The comparator 18 compares the data from the storage circuit 20 with the data to be stored next time from the magnetic sensor 17, and outputs an operation signal when the difference exceeds a preset value. This can be illustrated as shown in A and B in Figure 2. The operating signal is sent to the synchronization circuit 16. A vertical synchronization signal is separately input to the synchronization circuit 16, and from these two signals, a period signal spanning a preset number of vertical scanning periods (two periods in D in FIG. 2) is generated.

The period signal is supplied to a gate circuit 8, a display blanking circuit 19, and a charging switch 15, which constitute an alternating drive circuit 21. The gate circuit 8 passes the alternating signal of a predetermined frequency from the oscillator 7 to the amplifier 9 for the duration of the period signal, where it is amplified. The output of amplifier 9 is 2
One branch is applied to the base of transistor 11, and the other is inverted in polarity by polarity inverter 10 and applied to the base of transistor 12. transistor 1
1 and 12 have a potential arrangement such that when the base becomes positive of the alternating signal, the collector and emitter become conductive, so in the end, they become conductive alternately every half cycle of the alternating signal. .

The collector of the transistor 11 is connected to the a-side terminal of the degaussing coil 13, and the collector of the transistor 12
The collector is connected to the b-side terminal of the degaussing coil 13. Also, the emitters of both transistors are connected to capacitor 1.
The center tap of the degaussing coil 13 is connected to the positive terminal of the capacitor 14. Therefore, if an alternating signal is applied to transistors 11 and 12 while the capacitor is charged, current will alternately flow through the degaussing coil 13 from the center tap in the direction of the arrow for each half cycle of the alternating signal. The same effect occurs when an alternating current is passed through the coil 13.

By the way, the capacitor 14 is connected to a DC power source via a charging switch 15, and a period signal is applied to the charging switch 15. Charging switch 15
The capacitor 14 is disconnected from the DC power supply during the period when the period signal exists, and the DC power supply is connected to the capacitor 14 when the period signal is not present.

Therefore, when a period signal is output from the synchronous circuit 16, an alternating signal is applied to the transistors 11 and 12 during that period, and the capacitor 14 is disconnected from the DC power supply, so that discharge begins. As a result, capacitor 1
The voltage of 4 decreases with the passage of time and becomes as shown in E of FIG.

As a result, the current flowing through the demagnetizing coil 13 becomes an attenuated pulsating current as shown in F and G in FIG. A damped alternating field is obtained to achieve the purpose of demagnetization. Further, since no image is displayed during this period, a blank signal as shown in FIG. 2 is outputted from the display blank circuit 19 in order to cut off the electron beam of the cathode ray tube.

In this embodiment, the time equivalent to two vertical scans is used for degaussing, but this is a sufficiently long time compared to the case where degaussing is performed only during the retrace period of the vertical scan, so the degaussing current is also sufficient. A large current can be passed, and even strongly magnetized shadow masks can be sufficiently demagnetized. On the other hand, even if there is an image blank for several cycles of vertical scanning, the human eye perceives it as only a momentary flicker, so it does not interfere with the operation of the display device at all.

[0025]

Effects of the Invention As explained above, the shadow mask degaussing circuit for a cathode ray tube display device of the present invention is capable of degaussing a predetermined number of vertical scanning cycles when the amount of change in the intensity of the external field exceeds a set value. Since the demagnetization operation is carried out over a period of time, there is an advantage that even strong magnetization can be sufficiently demagnetized by flowing a sufficiently large demagnetization current only when there is a magnetization strong enough to require demagnetization.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of an embodiment of a degaussing circuit of the present invention.

FIG. 2 is an operation waveform diagram of each part of the circuit according to the embodiment of the present invention.

FIG. 3 is a diagram showing the configuration of a conventional degaussing circuit.

[Explanation of symbols]

1 Cathode ray tube 2 Demagnetizing coil 3　Alternate power supply 4 Positive characteristic thermistor 5 Relay 6 Power on timer or manual switch 7 Oscillator 8 Gate circuit 9 Amplifier 10 Polarity inverter 11 Transistor 12 Transistor 13 Demagnetizing coil 14 Capacitor 15 Charging switch 16 Synchronous circuit 17 Magnetic sensor 18 Comparator 19 Display blank circuit 20 Memory circuit 21 Alternating drive circuit

Claims (1)

[Claims] [Claim 1] A magnetic sensor that detects the strength of an external magnetic field; Storage means that sequentially updates and stores the value of the magnetic field strength signal from the magnetic sensor at fixed time intervals; a comparator that compares the value of the magnetic field strength, calculates the difference, and generates an operating signal when the difference value exceeds a preset value; receives the operating signal and the vertical synchronization signal; a synchronizing circuit that outputs a period signal that lasts for a number of vertical scanning cycles; a display blanking circuit that outputs a blank signal for cutting off the electron beam of the cathode ray tube during the period according to the period signal; and a display blanking circuit that generates a demagnetizing magnetic field. a degaussing coil; an alternating drive circuit that receives a period signal and drives the degaussing coil alternately during the period; a capacitor that serves as a power source for an alternating current flowing through the degaussing coil; 1. A shadow mask degaussing circuit for a cathode ray tube display device, comprising: a charging switch that is connected to a DC power supply at a constant voltage during the period and disconnected from the DC power supply during the period;